The present invention relates to an oscillator and in particular, although not exclusively, to a Gunn diode oscillator susceptible to automated mass production. The present invention relates also to a method of assembling oscillators and to a method of tuning oscillators.
Gunn diode oscillators are important components of radar systems. They are formed from gallium arsenide substrates which are processed in a particular manner to form devices known as Gunn diodes. A Gunn diode is incorporated into an oscillator package to form an oscillator which can generate microwave frequency electromagnetic radiation. Application of a dc voltage across a Gunn diode causes high frequency electron pulses across junctions within it, which pulses cause an oscillating electric field to be set up in the vicinity of the Gunn diode. The Gunn diode is located in, and supported by, a metallic oscillator body which defines a waveguide, supports other components of the oscillator, and uses the electric field to generate microwave frequency electromagnetic radiation. A prior art bias-tuned, second harmonic, radial mode oscillator is shown in section in FIG. 1.
In FIG. 1, a Gunn diode oscillator comprises generally a body 10 in which is defined an elongate waveguide 11 of rectangular cross-section, a bore 12 containing an RF choke 13, and a composite bore 14 on the opposite side of the waveguide to the bore. A Gunn diode chip 15 is ultrasonically scrubbed onto the top surface of a gold-plated, first heat sink 16. The Gunn diode 15 is enclosed by an annular, electrically-insulating tube 17, which is made from alumina, and an electrically-conductive lid 18. A gold-plated radial disk 19 is held in contact with the lid 18 by application of a constant spring force on an electrically conducting-connector 20, which passes along the central axis of the RF choke 13. The surface of the Gunn diode 15 closest to the RF choke 13 is electrically connected to the connector 20 by way of a wire bond or Maltese cross connection 21 made to the junction of the alumina tube 17 and the lid 18.
The RF choke 13 comprises alternate high impedance and low impedance sections. In this example, the low impedance section is a brass disk 22, and the high impedance section is of air 23. The thickness of each of the sections 22 and 23 is equal to three quarters of the wavelength of the electromagnetic radiation to be generated. Further low and high impedance sections 22 and 23 may be incorporated in the choke 13. The connector 20, and thus the radial disk 19, is electrically insulated from the oscillator body 10 by a layer of plastics material (not shown) which is wrapped around the substantially cylindrical form of the choke 13. The surface of the Gunn diode 15 furthest from the RF choke 13 is electrically and thermally connected to the oscillator body 10 by the first heat sink 16 and a second heat sink 24. The Gunn diode oscillator can be activated to generate microwave frequency electromagnetic radiation by application of a dc voltage across the oscillator body 10 and the electrical connector 20.
A short circuit 25, in the form of an elongate rectangular cross section metallic element, is movable axially along the waveguide 11. Movement of the short circuit 25 relative to the position of the radial disk 19 and Gunn diode 15 causes variation of the output power of the oscillator, as will be appreciated by those skilled in the art. Microwave frequency electromagnetic energy is radiated in the direction of the arrow.
The first heat sink 16 is connected to the second heat sink 24 by the mating of a threaded axial bore 26 of the second heat sink 24 and a screw thread 27 formed on the primary surface of the first heat sink 16. This connection is made by rotation of the first heat sink 16 relative to the second heat sink 24, achieved by rotation of a screwdriver or the like having a blade inserted in a groove 28 formed in the end of the heat sink 16 furthest from the RF choke 13, to screw the heat sinks 16 and 24 together. Rotation of these components is made until the surface of a flange 29 of the first heat sink 16 which is furthest from the RF choke 13 is in firm contact with a surface of the second heat sink 24 which is closest to the choke 13. A reasonable degree of thermal and electrical contact is thus made between the first and second heat sinks 16 and 24.
The second heat sink 24, and thus the first heat sink 16, is supported in the oscillator body 10 by the clamping action of an annular screw 30. The screw 30 has a thread 31 on its outer surface which mates with a threaded bore 32 of the composite bore 14. The second heat sink 24 is fixed relative to the body 10 by virtue of frictional forces at the washer shaped contact areas 35 and 36. A flange 29 of the first heat sink 16 and a part of the second heat sink 24 which is adjacent the flange 29 extend through a bore 33 linking the bore 32 with the waveguide 11 so that the Gunn diode 15 is supported in and located in the waveguide 11.
Because it is difficult to predict what effect the mating of the threads 26 and 27 will have on the concentricity of the outer cylindrical surface of the second heat sink 24 and the central axis of the first heat sink 16, it is usual to provide the bore 33 with a diameter 3 or 4% larger than the outer diameter of the flange 29. It has been found that the unscrewing of the annular screw 30 and subsequent re-screwing can cause the operating characteristics of the Gunn diode oscillator to change. In addition to a significant change in the level of the output power, a frequency change of up to 2% can occur as a result of such a re-assembly. This obviously introduces some undesirable variables into the characteristics that can be expected on oscillator assembly, even when tight component tolerances can be achieved. This frequency pulling and the changing of the output power has been thought to be caused by different alignment of the bond wires or Maltese cross connections 21 with respect to the axis of the waveguide 11. However, the inventor has found that the frequency pulling and output power change caused by the re-assembly is the result largely of the outer surface of the flange 29, and thus the outer surface of the part of the second heat sink 24 which is closest to the choke 13, being either more or less concentric with the bore 33 than previously. The concentricity has a bearing on the shape of the air gap between the outer surface of the heat sink 24 and the bore 33 and particularly on the minimum distance between the bore 33 and the outer surface of the heat sink 24. The inventor has performed experiments which show that the amount of frequency pulling is dependent on the amount of offset from concentric of the bore 33 and the outer surface of the flange 29 of the heat sink 16.
In accordance with a first aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation comprising:
an oscillator body having formed therein a waveguide and a substantially cylindrical bore intersecting the waveguide; and
a package comprising a heat sink having a substantially cylindrical portion and an oscillatory semiconductor device supported at one end of the heat sink;
in which the diameter of the cylindrical portion of the heat sink in relation to the diameter of the bore is such that the heat sink is supported in the bore by one of a) an interference fit, and b) a tight sliding fit, with the semiconductor device being located in the waveguide.
This oscillator is not susceptible to variations in frequency and output power which can occur due to variations in concentricity of the heat sink with the bore in which it is supported. This oscillator is also likely to be cheaper to produce than the prior art oscillators because it is not necessary to form the heat sink package with a threaded outer surface, or to form a second heat sink with a threaded bore. This oscillator is also likely to have greater surface area contact, because of the lack of threaded connections and because contact between the heat sink and the body can be made for all or much of the cylindrical outer surface of the heat sink, than the prior art oscillators. This increased contact will improve the thermal and electrical conductivity properties and is likely to improve the operating characteristics of the oscillator. This oscillator is more readily susceptible to automated mass production than the prior art oscillators because the controlled insertion of the package into the bore in the body is easily achievable by a robotic tool. Also, because the possibility that the package will move relative to the body may be substantially reduced, the possibility that the characteristics of the oscillator will change as a result of a mechanical knock or thermal cycling may also be reduced.
The interference or tight sliding fit preferably includes a layer of soft deformable material. The soft deformable material layer provides the advantage of lubricating the interference fitting of the package and the bore. The package may be barrel plated with soft gold to provide the layer. The plating may for example be 15 xcexcm thick.
A surface of the heat sink adjacent the diode is preferably substantially flush with or slightly proud of a surface of the waveguide. This feature may ease mass production of the oscillator in that protrusion of the heat sink into the waveguide can be detected optically or mechanically, the detection being used to control the insertion of the package into the body.
The bore may be formed by the reaming of a rough bore formed in the body. This allows the diameter of the bore to be very accurately determined, thus assisting the interference fit. The rough bore may be formed by die casting the body. Cost savings can be achieved by allowing the body to be die cast in this way, prior art second harmonic oscillator bodies not being thought to be susceptible to die casting.
The body may comprise a second bore on the opposite side of the first bore to the waveguide, the second bore being threaded to allow a screw cap to retain the package within the oscillator body. This feature eliminates any possibility that the Gunn diode package will be dislodged by the spring force of the RF choke.
In accordance with a second aspect of the present invention, there is provided a method of assembling an oscillator for generating microwave frequency radiation, the oscillator comprising:
an oscillator body having formed therein a waveguide and a substantially cylindrical bore intersecting the waveguide; and
a package comprising:
a heat sink having a substantially cylindrical portion, and
an oscillatory semiconductor device supported at one end of the heat sink;
in which the diameter of the cylindrical portion of the heat sink is larger than the diameter of the bore,
the method comprising inserting the package into the bore such that the package is retained by one of a) an interference fit, and b) a tight sliding fit, with the semiconductor device being located in the waveguide.
The first heat sink 16 is connected to the second heat sink 24 by the mating of a threaded axial bore 26 of the second heat sink and a screw thread 27 formed on the primary surface of the first heat sink. The second heat sink 24, and thus the first heat sink 16, is supported in the oscillator body 10 by the clamping action of an annular screw 30. The screw 30 has a thread 31 on its outer surface, which thread mates with a threaded bore 32 of the composite bore 14. A flange 29 of the first heat sink 16 and a part of the second heat sink 24 which is adjacent to the flange 29 extend through a bore 33 linking the bore 32 with the waveguide 11, so that the Gunn diode 15 is supported in, and located by the waveguide 11.
In setting up the oscillator so that it has the desired characteristics, the oscillator is assembled so that it has the structure thus far described. A short circuit 50 is inserted manually into the waveguide 11 by manipulation of a rod 51 which is temporarily screwed into a bore formed in an end of the short circuit which is furthest from the Gunn diode 15. The short circuit 50 comprises a machined brass component which has a rectangular cross-section of dimensions sufficiently less than the dimensions of the cross-section of the waveguide 11 to allow the short circuit to be moved along the longitudinal axis of the waveguide without encountering significant mechanical resistance from the walls of the waveguide.
The displacement of the face of the short circuit 50 closest to the Gunn diode 15 along the waveguide 11 determines the output power of the oscillator in the relationship illustrated in FIG. 2. The curve 80 relates to a Gunn diode voltage of 6.5V, and the curve 81 relates to a Gunn diode voltage of 4.5V. The short circuit 50 is moved toward the Gunn diode 15, from a position approximately three quarters of a wavelength of the radiation from the position of the Gunn diode by manipulation of the rod 51 until the output power of the oscillator is equal to a desired value. A desired value will typically be 50 mW, which is about two thirds of the maximum output power. When the position of the short circuit 50 is such that the output power level is at the desired power level, a screw 52 is rotated to lock the short circuit in place. The rod 51 may then be unscrewed and removed from the short circuit 50.
However, it has been found that the output power of the oscillator can vary quite considerably from the relationship shown in FIG. 2, and that there may be a number of different output power levels which can be measured for a single given displacement of the short circuit 50. Also, it has been found that the output power level can change as a result of the screw 52 being used to lock the short circuit 50 in position in the waveguide 11.
Such variation of the output power is undesirable, and is likely to hinder the setting up process if the process is automated. Automated setting up is complicated also by the fact that the rod 51 and the screw 52 will require separate robotic tools to operate at the same time.
In accordance with a third aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation, the oscillator comprising:
an oscillator body having formed therein a waveguide;
an oscillatory semiconductor device supported in the waveguide; and
a short circuit having a skin of soft metal on at least one outer surface thereof;
the short circuit having dimensions selected such that the skin allows the short circuit to be supported in the waveguide by one of a) a tight sliding fit, and b) an interference fit.
This invention arose, in part, as a result of determining that the power level variations in the above described prior art oscillator is caused by movement of the short circuit 50 other than longitudinally in, and particularly vertically in, the waveguide 11; and the location of, and extent of, electrical contact between the oscillator body 10, the short circuit 50 and the screw 52.
An oscillator constructed in accordance with this aspect of the present invention will not experience variations in output power level caused by movement of the short circuit, other than in a longitudinal direction, because the short circuit is firmly supported in the waveguide by the tight sliding or interference fit. Automated oscillator set-up may be facilitated, both by the reduced number of robotic tools needed for simultaneous operation, and particularly by the avoidance of power level variations. An oscillator constructed in accordance with this aspect of the present invention may also be more mechanically rugged, in that a tight sliding or interference fit may be more resistant to mechanical knocks than the lock-screw arrangement of the prior art. Mechanical integrity is of particular importance where correct operation of the oscillator is important for safety, such as in, for example, automotive autonomous cruise control applications. Here, the oscillator is likely to be subjected to wide temperature variations and to mechanical vibration.
The cost of the components for an oscillator constructed in accordance with this invention is likely to be lower than the cost of the components of the prior art oscillator described above, as there is no longer the need for a threaded bore to support the screw 52, or for the screw 52 itself. Also, a circular cross-section short circuit is likely to be less expensive to manufacture than the prior art rectangular cross-section short circuit especially so in volume production.
The transition between the first and section portions of the waveguide is preferably a step transition. Such an arrangement may be easier to machine than other possible arrangements, which may therefore reduce manufacturing costs.
The transition is preferably at a position which is chosen to provide the oscillator with a smooth power tuning characteristic.
In accordance with a fourth aspect of the present invention, there is provided a method of assembling an oscillator for generating microwave frequency radiation, the oscillator comprising: an oscillator body having formed therein a waveguide, an oscillatory semiconductor device supported in the waveguide; and a short circuit having a skin of soft metal on at least one outer surface thereof; in which the cross-sectional dimensions of the short circuit with the skin are larger than the cross-sectional dimensions of the waveguide, the method comprising inserting the short circuit into the waveguide such that the short circuit is retained in the waveguide by one of a) an interference fit, and b) a tight sliding fit.
Although coarse tuning of the oscillator frequency is performed prior to the output power of the oscillator being set, fine tuning is performed afterward. Where the oscillator is to be used in an automotive autonomous cruise control application, it may be desired for the oscillator to be swept from 76.1 GHz to 76.9 GHz. Such a sweep may be obtained by ramping the dc voltage applied to the Gunn diode from 4.5 volts to 6.0 volts. It is common with such oscillators to find that, due to variations in the mechanical arrangement of the package 16, 17, 18 and variations in the GaAs Gunn diode itself, the frequency of radiation obtained from a 6 volt supply can vary by up to 3 GHz either side of the desired frequency.
Coarse tuning is achieved by the replacement of the RF choke and radial disk assembly with an assembly having a disk of an appropriate diameter. During the coarse tuning, the oscillator is set up so that a frequency between 77 and 78 GHz is obtained when 6.0 volts is applied across the Gunn diode. An inventory of, typically, seven choke and disk assemblies, each of different disk diameter, will be necessary to obtain a coarse-tuned frequency sufficiently near to the desired frequency to allow fine tuning. Fine tuning is achieved by insertion of a frequency tuning probe into the volume between the radial disk 19 and the heat sink 16. The presence of the frequency tuning probe in this volume, because the material from which it is made has different dielectric properties to the air in the volume, disturbs the electric field between the radial disk 19 and the heat sink 16. The disturbance of the electric field causes the oscillation frequency of the radial mode oscillator, and thus the frequency of the radiation generated, to change. The extent of the change in frequency is dependent on, in particular, the extent of protrusion of the probe into the volume, the dimensions of the probe and the material used to make the probe.
It has been known for a probe made from either metal or dielectric, having a cylindrical form, and having a diameter of 0.2 to 0.3 mm, to be used to tune oscillators of the type described. Such a probe allows a reduction of the oscillator frequency as the probe is moved towards the tube 17. Because such probes act directly on the radial mode oscillator, the operating frequency is hypersensitive to their position. Large amounts of frequency change are often obtained from a relatively small movement of the probe, which can make fine tuning of the oscillator frequency difficult. Further problems are experienced because the small size of the probes make them difficult to manufacture, handle and support.
It has been found also that the presence of the probe in the radial mode oscillator can cause unpredictable fluctuations in the output power of the oscillator. This clearly is undesirable.
In accordance with a fifth aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation the oscillator comprising:
an oscillator body;
a waveguide formed in the oscillator body;
an oscillator package supporting an oscillatory semiconductor device in the waveguide, the oscillator package providing mechanical protection for the oscillatory semiconductor device;
a conduit formed in the oscillator body, the conduit intersecting the waveguide;
a choke assembly supported in the conduit;
a radial disk which is mechanically connected to the choke assembly and which is urged by the choke assembly to be in contact with the oscillator package in the waveguide; and
a frequency tuning probe which is arranged to be movable into the coaxial resonator volume between the radial disk and the choke assembly to alter the frequency of the radiation generated by the oscillator.
In accordance with a sixth aspect of the present invention, there is provided a method of tuning an oscillator for generating microwave frequency radiation, the oscillator comprising: an oscillator body; a waveguide formed in the oscillator body; an oscillator package supporting an oscillatory semiconductor device in the waveguide, the oscillator package providing mechanical protection for the oscillatory semiconductor device; a conduit formed in the oscillator body, the conduit intersecting the waveguide; a choke assembly supported in the conduit; a radial disk which is mechanically connected to the choke assembly and which is urged by the choke assembly to be in contact with the package in the waveguide, the method comprising moving a frequency tuning probe into the volume between the radial disk and the choke assembly.
Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings.